This invention relates to plasma excitation recombination light sources such as lasers.
In Applied Physics Letters, Vol. 36, No. 8, p. 615 (1980), W. T. Silfvast, L. H. Szeto and O. R. Wood, II, describe a new electric discharge device developed for producing laser action in the atomic spectra of a number of metal vapors by a segmented plasma excitation and recombination (SPER) mechanism. This laser includes a number of narrow metal strips (of the lasing species) positioned end-to-end on an insulating substrate in such a way as to leave a small gap between each pair of adjacent strips. The strips are positioned in a laser cavity containing either a buffer gas (preferably) or a vacuum and typically are 1 mm thick, 2 mm wide, and 10 mm long (hereinafter "bulk electrodes"). When a high-voltage, high-current pulse is applied to the end strips of this arrangement, a high-density metal-vapor ion plasma is formed in each gap. Once formed, these plasmas (consisting primarily of vaporized strip material) expand essentially hemispherically, cool in the presence of the background gas (e.g., helium) at low pressure and recombine. Using this laser configuration, laser action was observed in the near infrared at more than seventy wavelengths between 0.29 and 3.95 .mu.m in 11 elements (Ag, Bi, C, Ca, Cd, Cu, In, Mg, Pb, Sn, Zn), 3 of which (Mg, Zn, In) had not been observed to oscillate in their neutral spectrum before. Some of these results are reported in the aforementioned APL article; others are reported by W. T. Silfvast et al in Applied Physics Letters, Vol. 39, No. 3, p. 212 (1981) and in Optics Letters Vol. 7, No. 1, p. 34 (1982).
The SPER laser is simple to construct, can be easily scaled in length and volume, has been shown to be capable of long life, and has the potential for high efficiency. It is the subject matter of copending application Ser. No. 82,308 filed on Oct. 5, 1979, now U.S. Pat. No. 4,336,506 issued on June 22, 1982, and copending application Ser. No. 367,092 filed on Apr. 9, 1982, now U.S. Pat. No. 4,395,770 issued on July 26, 1983, both of which are assigned to the assignee hereof.
Lasing action in a SPER laser is not observed with equal facility with all metals, even at high pressure of the background gas. A figure of merit, M, can be derived which defines the relative ease of achieving lasing action in a metal vapor. M is defined as follows: EQU M=1/KcpT.sup.2 ( 1)
where k is the thermal conductivity of the metal, c is the specific heat of the metal, p is the density of the metal, and T is the absolute temperature of the surface of the metal electrode. This expression is derived from a heat equation for the time necessary to raise the electrode surface temperature to a value T by ion or electron current entering the electrode surface. For the value of the temperature at which the vapor pressure of the metal is approximately 0.1 Torr, a list of a number of metals together with calculated values of M normalized to the value for Cd metal, is given in the following table:
______________________________________ METAL M ______________________________________ Cd 1.00 Na .99 Pb .57 In .41 Zn .39 Li .33 Ca .27 Sn .11 Ni .03 Al .02 Cu .01 ______________________________________
For the alkali metals (Na, Li, Ca) M should be calculated for the oxides that form for these metals. Experimentally, metals with M.about.1 have been found to easily produce segmented metal vapor plasmas necessary for lasing action in SPER lasers at low background gas pressures (e.g., 1-10 Torr), whereas metals with M&lt;&lt;1, such as Li, Al, Ca, and Cu, do not even produce segmented plasmas. With these metals, as the background gas pressure is reduced, the discharge current is carried by a discharge in the background gas between non-adjacent electrodes, effectively shorting out the intervening metal-vapor arcs, reducing the number of metal vapor plasmas and, hence, lowering the net gain.